Does a source of electricity ever run out of electrons?
Category: Physics Published: March 17, 2016
By: Christopher S. Baird, author of The Top 50 Science Questions with Surprising Answers and Associate Professor of Physics at West Texas A&M University
The answer to this question depends on the situation. We can roughly classify all electrical systems into two categories: static electricity systems and circuit electricity systems. Note that all electrical effects are actually part of one unified set of physical laws. This classification is therefore ultimately arbitrary and over-simplified. However, this classification is sufficient for our current purpose of understanding electric current.
A static electricity system involves the flow of electric current as a result of a buildup of electric charge somewhere. Such a system does not involve a closed electrical circuit. Examples of this type of system include lightning and the sparks you get when you rub your feet on a carpet. Electrons naturally repel each other. When a lot of electrons get piled up in one place, they can push on each other so strongly that some of the electrons get pushed right off of the object. They end up getting pushed out through the air, the water, or whatever surrounds the object. We call a collection of moving electrons an electric current, therefore a buildup of charge can drive a current. The electrons simply flow away from the pile and ultimately end up attached to atoms in the environment. In this way, we can have an electric current even if we don't have a complete electrical circuit. In air, an electrical current takes the form of dark discharge, corona discharge, or sparks (depending on if the current is weak, medium strength, or strong, respectively). Note that the name "static electricity" is a poor name since the electric charge is not always stationary in this type of system. More accurate names would be "non-circuit electricity" or "charge buildup electricity."
Since charge buildup is the cause of the electric current in static electricity systems, the current will stop flowing once the buildup goes away. As the electrons flow away from the pile, the pile gets smaller. Eventually, the pile of excess electrons is gone (the electrons that are needed to keep the molecules neutral still remain, but they don't do much). Quite literally, electricity stops flowing because the source runs out of excess electrons. This is why lightning bolts and the sparks between statically-charged socks go away quickly. It's not that electrons are destroyed. Rather, they are leaked away to distant points until none remain.
In contrast, circuit electricity systems involve the flow of electric current through a closed loop. This current is the result of a charge pump operating somewhere in the loop. This pump is also called a voltage source and can take the form of a battery, a solar cell, a generator, or the cord from a power grid. The pump creates a voltage difference along the circuit which drives charges like electrons through the circuit. The pump can either constantly pump electrons in one direction, which leads to a direct current (DC), or it can periodically switch off the direction in which it is pumping electrons, which leads to an alternating current (AC). For simplicity, let's focus on direct current.
As the electrons flow through the circuit, they flow down the potential energy slope that is created by the voltage. Once they reach the pump at the end of the circuit, the low-energy electrons are boosted back up to a high potential energy so that they can start flowing through the circuit again. The situation is a bit like an artificial waterfall in your backyard. Water flows down the waterfall and into a pool because of the natural pull of gravity, just like how electrons flow through the circuit because of the pull of the applied voltage. A water pump then pushes the water in the pool back up to a high energy state at the top of the waterfall, just like how a battery pushes electrons back up to a higher energy state at the beginning of the circuit. The cycle then repeats.
Since the pumping of charge is the cause of the electric current in a circuit electricity system, the current will never stop flowing as long as the pump remains on and the circuit remains uninterrupted. Circuits don't create, destroy, use up, or lose electrons. They just carry the electrons around in circles. For this reason, circuit electrical systems can't really run out of electrons. The energy delivered through a circuit is not the result of electrons existing in the circuit. Electrons always exist in the circuit as part of the atoms and molecules that make up the circuit. The electrical energy that is delivered is the result of the electrons moving through the circuit. Turn off the pump (i.e. disconnect the battery), and the electrons stop moving through the circuit. But the electrons don't go away. They are still there as a natural part of the materials in the circuit.
As I said before, the categorization of systems into static and circuit systems is somewhat arbitrary and oversimplified. Real electrical systems contain a combination of both effects. For instance, a circuit often contains a capacitor. While the circuit acts overall like a circuit electricity system, the capacitor acts more like a static electricity system. As a result, a capacitor can indeed run out of electrons. As soon as one side of the capacitor is depleted of electrons, the electric current stops flowing through the capacitor. At that moment, the part of the circuit containing the capacitor switches from acting like a circuit electricity system to a static electricity system. This happens in the sense that current is now being stopped by a lack of electrons, and not by the lack of an electron pump or the lack of a complete circuit.